1. Field of the Invention
The present invention relates generally to semiconductor devices and manufacturing methods thereof, and more specifically, to a semiconductor device capable of high performance with reduced contact resistance with a wiring structure used in the device and a method of manufacturing such a semiconductor device.
2. Description of the Background Art
In recent years, silicon thin films are used as materials for wiring and electrodes in silicon semiconductor devices. This is because a silicon thin film is thermally stable, its resistance can be reduced by implanting impurity such as phosphorus, arsenic, and boron, and deposition/treatment of the film is relatively easy.
As the integration densities of silicon semiconductor devices increase, however, the width of a wiring layer is reduced and even a silicon thin film implanted with impurity suffers from its wiring resistance.
Accordingly, a refractory metal silicide/silicon thin film structure in which a refractory metal silicide layer is deposited on a silicon thin film is often used.
The refractory metal silicide used for the structure includes tungsten silicide, molybdenum silicide, nickel silicide, cobalt silicide, and titanium silicide. Tungsten, molybdenum, nickel, cobalt, titanium and the like are used for their thermal stability, resistance smaller than an impurity implanted silicon thin film, little mutual diffusion when used in upper and lower layers of a wiring layer, and stability when formed into a stacked layer structure.
As described above, a refractory metal silicide/silicon thin film structure is used as a low resistance wiring material. In a silicon semiconductor device, however, in addition to the wiring layer, there are a number of overlying layers present with insulating films therebetween.
In such a structure, contact resistance between upper layer wiring and lower layer wiring is of great significance. More specifically, unlike a conventional single layer of silicon thin film, if a refractory metal silicide is present on a wiring layer, contact resistance with the overlying refractory metal silicide will be higher than contact resistance with a conventional silicon thin film.
This is because a native oxide film, removal of which is not easy, is likely to form on the refractory metal silicide and contact resistance increases as a result.
A technique for solving this problem is disclosed in, for example, Japanese Patent Laying-Open No. 2-32537.
According to this technique, in order to remove an oxide film formed on the surface of a refractory metal silicide, the semiconductor substrate is subjected to a heat treatment at 800.degree. C. in a hydrogen atmosphere.
This heat treatment removes the native oxide film and reduced contact resistance results.
The above-described prior art technique however suffers from the following problems.
Firstly, as described above, the native oxide film is removed away by performing a heat treatment at 800.degree. C. in a hydrogen atmosphere.
However, since the native oxide film formed on the refractory metal silicide layer cannot be removed completely, as illustrated in a cross section in FIG. 27, native oxide film 34 partially remains at the contact portion, and the contact resistance value cannot be lowered to a desired value.
Note that the cross section in FIG. 27 shows the contact portion to source/drain regions 14, 15 in an active region surrounded by the element isolation region 4 of a semiconductor substrate 2.
Formed on the surface of source/drain regions 14, 15 are a polysilicon layer 20 and a refractory metal silicide film 22. A silicon electrode 32 is connected to refractory metal silicide film 22 with an interlayer insulating film 24 therebetween. Unremoved part of native oxide film 34 remains on the surface of refractory metal silicide film 22.
Secondarily, refractory metal silicide film 22 removed of the silicon oxide film is still encountered with another problem. The crystal structure of refractory metal silicide film 22 is as illustrated in FIG. 28 an arrangement of large crystals 22a.
Accordingly, if a silicon thin film is formed on refractory metal silicide film 22, monosilane used for forming the silicon thin film passes through gaps between crystals 22a.
Therefore, referring to FIG. 29, a new silicon layer 32 is formed on silicon layer 20 under refractory metal silicide film 22. Thus, refractory metal silicide film 22 is lifted up and comes off from silicon layer 20.